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Publications in peer reviewed journals

13 Publications found

Mineral-Associated Soil Carbon is Resistant to Drought but Sensitive to Legumes and Microbial Biomass in an Australian Grassland

Canarini A, Mariotte P, Ingram L, Merchant A, Dijkstra FA

2017 - Ecosystems, e-publication, 1-15

Abstract:

Drought is predicted to increase in many areas of the world with consequences for soil carbon (C) dynamics. Plant litter, root exudates and microbial biomass can be used as C substrates to form organo-mineral complexes. Drought effects on plants and microbes could potentially compromise these relative stable soil C pools, by reducing plant C inputs and/or microbial activity. We conducted a 2-year drought experiment using rainout shelters in a semi-natural grassland. We measured aboveground biomass and C and nitrogen (N) in particulate organic matter (Pom), the organo-mineral fraction (Omin), and microbial biomass within the first 15 cm of soil. Aboveground plant biomass was reduced by 50% under drought in both years, but only the dominant C4 grasses were significantly affected. Soil C pools were not affected by drought, but were significantly higher in the relatively wet second year compared to the first year. Omin-C was positively related to microbial C during the first year, and positively related to clay and silt content in the second year. Increases in Omin-C in the second year were explained by increases in legume biomass and its effect on Pom-N and microbial biomass N (MBN) through structural equation modeling. In conclusion, soil C pools were unaffected by the drought treatment. Drought resistant legumes enhanced formation of organo-mineral complexes through increasing Pom-N and MBN. Our findings also indicate the importance of microbes for the formation of Omin-C as long as soil minerals have not reached their maximum capacity to bind with C (that is, saturation).

Abstract:

The occurrence of sugar alcohols is ubiquitous among plants. Physiochemical properties of sugar alcohols suggest numerous primary and secondary functions in plant tissues and are often well documented. In addition to functions arising from physiochemical properties, the synthesis of sugar alcohols may have significant influence over photosynthetic, respiratory, and developmental processes owing to their function as a large sink for photosynthates. Sink strength is demonstrated by the high concentrations of sugar alcohols found in plant tissues and their ability to be readily transported. The plant scale distribution and physiochemical function of these compounds renders them strong candidates for functioning as stress metabolites. Despite this, several aspects of sugar alcohol biosynthesis and function are poorly characterised namely: 1) the quantitative characterisation of carbon flux into the sugar alcohol pool; 2) the molecular control governing sugar alcohol biosynthesis on a quantitative basis; 3) the role of sugar alcohols in plant growth and ecology; and 4) consequences of sugar alcohol synthesis for yield production and yield quality. We highlight the need to adopt new approaches to investigating sugar alcohol biosynthesis using modern technologies in gene expression, metabolic flux analysis and agronomy. Combined, these approaches will elucidate the impact of sugar alcohol biosynthesis on growth, stress tolerance, yield and yield quality.

Increased temperature causes different carbon and nitrogen processing patterns in two common intertidal foraminifera (Ammonia tepida and Haynesina germanica)

Wukovits J, Enge AJ, Wanek W, Watzka M, Heinz P

2017 - Biogeosciences, 11: 2815-2829

Abstract:

Benthic foraminifera are highly abundant heterotrophic protists in marine sediments, but future environmental changes will challenge the tolerance limits of intertidal species. Metabolic rates and physiological processes in foraminifera are strongly dependent on environmental temperatures. Temperature-related stress could therefore impact foraminiferal food source processing efficiency and might result in altered nutrient fluxes through the intertidal food web. In this study, we performed a laboratory feeding experiment on Ammonia tepida and Haynesina germanica, two dominant foraminiferal species of the German Wadden Sea/Friedrichskoog, to test the effect of temperature on phytodetritus retention. The specimens were fed with C-13 and N-15 labelled freeze-dried Dunaliella tertiolecta (green algae) at the start of the experiment and were incubated at 20, 25 and 30 degrees C respectively. Dual labelling was applied to observe potential temperature effects on the relation of phytodetrital carbon and nitrogen retention. Samples were taken over a period of 2 weeks. Foraminiferal cytoplasm was isotopically analysed to investigate differences in carbon and nitrogen uptake derived from the food source. Both species showed a positive response to the provided food source, but carbon uptake rates of A. tepida were 10-fold higher compared to those of H. germanica. Increased temperatures had a far stronger impact on the carbon uptake of H. germanica than on A. tepida. A distinct increase in the levels of phytodetrital-derived nitrogen (compared to more steady carbon levels) could be observed over the course of the experiment in both species. The results suggest that higher temperatures have a significant negative effect on the carbon exploitation of H. germanica. For A. tepida, higher carbon uptake rates and the enhanced tolerance range for higher temperatures could outline an advantage in warmer periods if the main food source consists of chlorophyte phytodetritus. These conditions are likely to impact nutrient fluxes in A. tepida/H. germanica associations.

Global patterns of phosphatase activity in natural soils

Abstract:

Soil phosphatase levels strongly control the biotic pathways of phosphorus (P), an essential element for
life, which is often limiting in terrestrial ecosystems. We investigated the influence of climatic and soil
traits on phosphatase activity in terrestrial systems using metadata analysis from published studies.
This is the first analysis of global measurements of phosphatase in natural soils. Our results suggest
that organic P (Porg), rather than available P, is the most important P fraction in predicting phosphatase
activity. Structural equation modeling using soil total nitrogen (TN), mean annual precipitation, mean
annual temperature, thermal amplitude and total soil carbon as most available predictor variables
explained up to 50% of the spatial variance in phosphatase activity. In this analysis, Porg could not be
tested and among the rest of available variables, TN was the most important factor explaining the
observed spatial gradients in phosphatase activity. On the other hand, phosphatase activity was also
found to be associated with climatic conditions and soil type across different biomes worldwide. The
close association among different predictors like Porg, TN and precipitation suggest that P recycling is
driven by a broad scale pattern of ecosystem productivity capacity.

Microbial utilization of mineral-associated nitrogen in soils

Abstract:

In soils, a large portion of organic nitrogen (ON) is associated with minerals and thus, possibly stabilized against biological decay. We therefore tested if mineral-associated N is an important N source for soil microorganisms, and which soil parameters control its bioavailability. Microcosm experiments with mineral-associated organic matter, obtained as heavy fraction (HF) via density fractionation, and bulk soil from mineral topsoil of the Franz Josef chronosequence were conducted for 125 days. We examined the effects of O2 status, soil age (differences in mineralogical properties), as well as cellulose and phosphate additions on the turnover of mineral-associated N. Using a combination of activity measurements and quantitative PCR, microbial N transformation rates and abundances of N-related functional genes (amoA, narG, chiA) were determined. Similar or higher values for microbial N cycling rates and N-related functional abundances in the HF compared to bulk soil indicated that mineral-associated N provides an important bioavailable N source for soil microorganism. The turnover of mineral-associated N was mainly controlled by the O2 status. Besides, soil mineralogical properties significantly affected microbial N cycling and related gene abundances with the effect depending on the N substrate type (ON, NH4+ or NO3−). In contrast, cellulose or phosphate addition hardly enhanced microbial utilization of mineral-associated N. The results of our microcosm study indicate that mineral-associated N is highly bioavailable in mineral topsoils, but effects of the mineral phase differ between N cycling processes.

Short-term carbon input increases microbial nitrogen demand, but not microbial nitrogen mining, in a set of boreal forest soils

Abstract:

Abstract Rising carbon dioxide (CO2) concentrations and temperatures are expected to stimulate plant productivity and ecosystem C sequestration, but these effects require a concurrent increase in N availability for plants. Plants might indirectly promote N availability as they release organic C into the soil (e.g., by root exudation) that can increase microbial soil organic matter (SOM) decomposition (‘‘priming effect’’), and possibly the enzymatic breakdown of N-rich polymers, such as proteins, into bio-available units (‘‘N mining’’). We tested the adjustment of protein depolymerization to changing soil C and N availability in a laboratory experiment. We added easily available C or N sources to six boreal forest soils, and determined soil organic C mineralization, gross protein depolymerization and gross ammonification rates (using 15N pool dilution assays), and potential extracellular enzyme activities after 1 week of incubation. Added C sources were 13C-labelled to distinguish substrate from soil derived C mineralization. Observed effects reflect short-term adaptations of non-symbiotic soil microorganisms to increased C or N availability. Although C input promoted microbial growth and N demand, we did not find indicators of increased N mobilization from SOM polymers, given that none of the soils showed a significant increase in
protein depolymerization, and only one soil showed a significant increase in N-targeting enzymes. Instead, our findings suggest that microorganisms immobilized
the already available N more efficiently, as indicated by decreased ammonification and inorganic N concentrations.
Likewise, although N input stimulated ammonification, we found no significant effect on protein depolymerization. Although our findings do not rule out in general that higher plant-soil C allocation can promote microbial N mining, they suggest that such an effect can be counteracted, at least in the short term, by increased microbial N immobilization, further aggravating plant N limitation.

Abstract:

Soil fluxomics analysis can provide pivotal information for understanding soil biochemical pathways and their regulation, but direct measurement methods are rare. Here, we describe an approach to measure soil extracellular metabolite (amino sugar and amino acid) concentrations and fluxes based on a 15N isotope pool dilution technique via liquid chromatography and high-resolution mass spectrometry. We produced commercially unavailable 15N and 13C labeled amino sugars and amino acids by hydrolyzing peptidoglycan isolated from isotopically labeled bacterial biomass and used them as tracers (15N) and internal standards (13C). High-resolution (Orbitrap Exactive) MS with a resolution of 50 000 allowed us to separate different stable isotope labeled analogues across a large range of metabolites. The utilization of 13C internal standards greatly improved the accuracy and reliability of absolute quantification. We successfully applied this method to two types of soils and quantified the extracellular gross fluxes of 2 amino sugars, 18 amino acids, and 4 amino acid enantiomers. Compared to the influx and efflux rates of most amino acids, similar ones were found for glucosamine, indicating that this amino sugar is released through peptidoglycan and chitin decomposition and serves as an important nitrogen source for soil microorganisms. d-Alanine and d-glutamic acid derived from peptidoglycan decomposition exhibited similar turnover rates as their l-enantiomers. This novel approach offers new strategies to advance our understanding of the production and transformation pathways of soil organic N metabolites, including the unknown contributions of peptidoglycan and chitin decomposition to soil organic N cycling.

Rhizospheric microbial community of Caesalpinia spinosa (Mol.) Kuntze in conserved and deforested zones of the Atiquipa fog forest in Peru

Cordero I, Ruiz-Diez B, Balaguer L, Richter A, Pueyo JJ, Rincon A

2017 - Applied Soil Ecology, 114: 132-141

Abstract:

Caesalpinia spinosa, tara, is the predominant fog catcher tree in the fog forest of Atiquipa, a biodiversity hotspot ecosystem within the coastal Peruvian desert highly threatened by intense land use over time. We investigated the impact of deforestation, as well as potential effects of the tree age (juveniles vs adults) and the type of tree (recruited vs planted), on the rhizospheric microbial communities of tara growing in contrasting landscapes (conserved vs deforested) of the Atiquipa forest.

We used a phospholipid fatty acids analysis approach to study the microbial community associated with tara. Additionally, we isolated and sought for native rhizospheric bacteria with plant growth promoting (PGPR) traits to be used as potential inoculants for restoration projects.

Deforestation profoundly altered the chemical and biological fertility of soils. All rhizospheric microorganisms were clearly reduced in abundance by deforestation, while the age or the type of trees had no effects. Both, deforestation and tree age influenced the assemblage of microbial communities, which tightly correlated with soil pH and organic matter among other soil properties. Adult trees harboured similar microbial communities in conserved and deforested soils being potential reservoirs of native microorganisms in the degraded areas. Some selected bacterial strains showed high plant growth promoting abilities, and PGPR traits were related with the isolation source of bacteria. The knowledge about key factors structuring the rhizospheric microbiota of tara and the identification of high-performing PGPR strains, provide a solid framework to formulate inocula for their use in restoration programmes in the Atiquipa fog forest.

Abstract:

We investigated to which extent phytosiderophores (PS), released by grasses for the acquisition of iron, solubilize other metals in contaminated soils, and how this affects metal mobilization and uptake in wheat plants. A plant-based bioassay (‘RHIZOtest’) and batch extraction scheme were carried out for assessing metal mobilisation in soil, PS exudation and metal accumulation in wheat. Increased PS exudation was observed in Fe-deficient wheat, leading to enhanced Zn, Cu, Mn and Ni concentrations in wheat shoots on some soils. In contrast, plant Cd and Pb concentrations were not affected. Likewise, in the batch experiment, strongly increased extractable Cu, Ni and Zn concentrations were observed, in particular when 100 or 1000 μM PS were added. Our results suggest that Fe deficiency can enhance the accumulation of some metals in shoots of grass species. Although our results indicate that the risk of enhanced accumulation of Cd and Pb in Fe deficient wheat shoots is rather small, further experiments conducted on soil for the complete vegetation period would be needed to confirm this observation.

Soil carbon loss regulated by drought intensity and available substrate: A meta-analysis

Canarini A, Kiær LP, Dijkstra FA

2017 - Soil Biology and Biochemistry, 112: 90-99

Abstract:

Drought is one of the most important climate change factors, but its effects on ecosystems are little
understood. While known to influence soil carbon (C) cycling, it remains unresolved if altered rainfall
patterns induced by climate change will stimulate positive feedbacks of CO2 into the atmosphere. Using a
meta-analysis frame-work including 1495 observations from 60 studies encompassing a variety of
ecosystems and soil types, we investigated drought effects on respiration rates, cumulative respiration
during drying-rewetting cycles, metabolic quotient (qCO2), dissolved organic C (DOC), microbial biomass
and fungi to bacteria (F:B) ratios from laboratory and field experiments. We show that C-rich soils (>2%
organic carbon) increase CO2 release into the atmosphere after intense droughts, but that C-poor soils
show a net decline in C losses. We explain this self-reinforcing mechanism of climate change in C-rich
soils by: (i) high substrate availability that magnify bursts of CO2 release after drought events and (ii) a
shift in microbial community with increased loss of C per unit of biomass. These findings shed light on
important responses of soil CO2 emissions to drought, which could either offset or facilitate positive
feedbacks to global warming. Our results should be considered in global climate models, as even small
changes in soil CO2 emission have large repercussions for global warming.

Organic and inorganic nitrogen uptake by 21 dominant tree species in temperate and tropical forests

Liu M, Li C, Xu X, Wanek W, Jiang N, Wang H, Yang X

2017 - Tree Physiology, 11: 1515-1526

Abstract:

Evidence shows that many tree species can take up organic nitrogen (N) in the form of free amino acids from soils, but few studies have been conducted to compare organic and inorganic N uptake patterns in temperate and tropical tree species in relation to mycorrhizal status and successional state. We labeled intact tree roots by brief 15N exposures using field hydroponic experiments in a temperate forest and a tropical forest in China. A total of 21 dominant tree species were investigated, 8 in the temperate forest and 13 in the tropical forest. All investigated tree species showed highest uptake rates for NH4+ (ammonium), followed by glycine and NO3− (nitrate). Uptake of NH4+ by temperate trees averaged 12.8 μg N g−1 dry weight (d.w.) root h−1, while those by tropical trees averaged 6.8 μg N g−1 d.w. root h−1. Glycine uptake rates averaged 3.1 μg N g−1 d.w. root h−1 for temperate trees and 2.4 μg N g−1 d.w. root h−1 for tropical trees. NO3− uptake was the lowest (averaging 0.8 μg N g−1 d.w. root h−1 for temperate trees and 1.2 μg N g−1 d.w. root h−1 for tropical trees). Uptake of NH4+ accounted for 76% of the total uptake of all three N forms in the temperate forest and 64% in the tropical forest. Temperate tree species had similar glycine uptake rates as tropical trees, with the contribution being slightly lower (20% in the temperate forest and 23% in the tropical forest). All tree species investigated in the temperate forest were ectomycorrhizal and all species but one in the tropical forest were arbuscular mycorrhizal (AM). Ectomycorrhizal trees showed significantly higher NH4+ and lower NO3− uptake rates than AM trees. Mycorrhizal colonization rates significantly affected uptake rates and contributions of NO3− or NH4+, but depended on forest types. We conclude that tree species in both temperate and tropical forests preferred to take up NH4+, with organic N as the second most important N source. These findings suggest that temperate and tropical forests demonstrate similar N uptake patterns although they differ in physiology of trees and soil biogeochemical processes.

Optimal metabolic regulation along resource stoichiometry gradients

Abstract:

Most heterotrophic organisms feed on substrates that are poor in nutrients compared to their
demand, leading to elemental imbalances that may constrain their growth and function. Flexible
carbon (C)-use efficiency (CUE, C used for growth over C taken up) can represent a strategy to
reduce elemental imbalances. Here, we argue that metabolic regulation has evolved to maximise
the organism growth rate along gradients of nutrient availability and translated this assumption
into an optimality model that links CUE to substrate and organism stoichiometry. The optimal
CUE is predicted to decrease with increasing substrate C-to-nutrient ratio, and increase with
nutrient amendment. These predictions are generally confirmed by empirical evidence from a new
database of c. 2200 CUE estimates, lending support to the hypothesis that CUE is optimised
across levels of organisation (microorganisms and animals), in aquatic and terrestrial systems, and
when considering nitrogen or phosphorus as limiting nutrients.

Abstract:

Predicted changes in the intensity and frequency of climate extremes urge a better mechanistic understanding of the
stress response of microbially mediated carbon (C) and nutrient cycling processes. We analyzed the resistance and
resilience of microbial C, nitrogen (N), and phosphorus (P) cycling processes and microbial community composition
in decomposing plant litter to transient, but severe, temperature disturbances, namely, freeze-thaw and heat. Disturbances
led temporarily to a more rapid cycling of C and N but caused a down-regulation of P cycling. In contrast to the
fast recovery of the initially stimulated C and N processes, we found a slow recovery of P mineralization rates, which
was not accompanied by significant changes in community composition. The functional and structural responses to
the two distinct temperature disturbances were markedly similar, suggesting that direct negative physical effects and
costs associated with the stress response were comparable. Moreover, the stress response of extracellular enzyme
activities, but not that of intracellular microbial processes (for example, respiration or N mineralization), was
dependent on the nutrient content of the resource through its effect on microbial physiology and community
composition. Our laboratory study provides novel insights into the mechanisms of microbial functional stress responses
that can serve as a basis for field studies and, in particular, illustrates the need for a closer integration of
microbial C-N-P interactions into climate extremes research.